In order to operate modern internal combustion engines and to comply with strict limiting values for emissions, an engine controller determines, by means of what is referred to as the cylinder filling model, the mass of air enclosed in a cylinder per working cycle. In accordance with the modeled air mass and the desired ratio between the air quantity and fuel quantity (Lambda) the corresponding fuel quantity setpoint value (MFF_SP) is injected by means of an injection valve, which is also referred to as an injector in this document. The fuel quantity to be injected can therefore be dimensioned in such a way that an optimum value for lambda is present for the exhaust gas post-treatment in the catalytic converter. For direct-injection spark-ignition engines with internal mixture formation, the fuel is injected directly into the combustion chamber at a pressure in the range from 40 to 200 bar.
A main requirement made of the injection valve is, along with leaktightness to prevent uncontrolled outflow of fuel and preparation of the jet of the fuel to be injected, also precise measurement of a predefined setpoint injection quantity. In particular in the case of super-charged direct-injection spark-ignition engines, a very large quantity spread of the required fuel quantity is necessary. Therefore, a maximum fuel quantity MFF_max per working cycle has to be metered for the super-charged mode at the full load of the engine, for example, whereas during operation near to idling conditions a minimum fuel quantity MFF_min has to be metered. The two characteristic variables MFF_max and MFF_min define here the limits of the linear working range of the injection valve. This means that for these injection quantities there is a linear relationship between the electric actuation duration (Ti) and the injected fuel quantity per working cycle (MFF).
The quantity spread, which in the case of a constant fuel pressure is defined as the quotient between the maximum fuel quantity MFF_max and the minimum fuel quantity MFF_min, for direct injection valves with a coil drive is approximately 15. For future engines in which the emphasis is on carbon dioxide reduction, the cubic capacity of the engines is reduced and the rated power of the engine is maintained or even raised by means of corresponding engine super-charging mechanisms. The required maximum fuel quantity MFF_max therefore corresponds at least to the requirements made of an induction engine with a relatively large cubic capacity. However, the minimum fuel quantity MFF_min is determined by means of operation close to idling conditions and the minimum air mass in the overrun mode of the engine with a decreased cubic capacity, and is therefore reduced. In addition, direct injection permits distribution of the entire fuel mass over a plurality of pulses, which permits more stringent limiting values for emissions to be complied with, for example in a catalytic converter heating mode by virtue of what is referred to as mixture stratification and a later ignition time. For the reasons mentioned above, future engines will be subject to increased requirements both in terms of the quantity spread and the minimum fuel quantity MFF_min.
In the case of known injection systems, a significant deviation of the injection quantity from the nominal injection quantity occurs at injection quantities which are less than MFF_min. This systematically occurring deviation can be attributed essentially to fabrication tolerances at the injector as well as to tolerances of the output stage, which actuates the injector, in the engine controller, and therefore to deviations from the nominal actuation current profile.
The electric actuation of a direct injection valve is typically carried out by means of a current-regulated full-bridge output stage. Under the peripheral conditions of application in a vehicle, only limited accuracy of the current profile with which the injector is supplied can be achieved. The resulting variation in the actuation current and the tolerances at the injector have significant effects on the achievable accuracy of the injection quantity, in particular in the range of MFF_min and below.
The characteristic curve of an injection valve defines the relationship between the injected fuel quantity MFF and the time period or the injection time Ti of the electric actuation as well as of the fuel pressure FUP (MFF=f(Ti,FUP)). The inversion of this relationship Ti=f−1 (MFF_SP,FUP) is used in the engine controller to convert the setpoint fuel quantity (MFF_SP) into the necessary injection time. The additional influencing variables, such as for example the cylinder internal pressure (Pcyl) during the injection process, fuel temperature (Θfuel) and possible variations of the supply voltage, which are input into this calculation, are omitted here for the sake of simplification.
FIG. 1 shows the characteristic curve of a direct injection valve. Here, the injected fuel quantity MFF is plotted as a function of the time period Ti of the electric actuation. As is apparent from FIG. 1, a working range which is linear to a very good approximation is obtained for the time periods Ti which are longer than Ti_min. This means that the injected fuel quantity MFF is directly proportional to the time period Ti of the electric actuation. A highly non-linear behavior occurs for time periods Ti which are shorter than Ti_min. In the illustrated example, Ti_min is approximately 0.5 ms.
The gradient of the characteristic curve in the linear working range corresponds to the static flow through the injection valve, i.e. the fuel throughflow rate which is achieved continuously in the case of a complete valve stroke. The cause of the non-linear behavior for time periods or injection times Ti which are shorter than approximately 0.5 ms or of fuel quantities MFF<MFF_min is, in particular, the inertia of an injector spring mass system and the chronological behavior during the building up and reduction of the magnetic field through a coil, which magnetic field activates the valve needle of the injection valve. As a result of these dynamic effects, the complete valve stroke is no longer achieved in what is referred to as the ballistic range. This means that the valve is closed again before the structurally predefined end position, which defines the maximum valve stroke, has been reached.
In order to ensure a defined and reproducible injection quantity, direct injection valves are usually operated in their linear working range. At present, operation in the non-linear range is not carried out since, owing to the abovementioned tolerances in the current profile and mechanical tolerances of injection valves (for example pretensioning force of the closing spring, stroke of the valve needle, internal friction in the armature/needle system), a significant systematic error occurs in the injection quantity. It becomes apparent from this that for reliable operation of an injection valve there must be a minimum fuel quantity MFF_min per injection pulse, which quantity has to be at least provided in order to be able to implement the desired injection quantity in a precisely quantified way. In the example illustrated in FIG. 1, this minimum fuel quantity MFF_min is somewhat smaller than 5 mg.
The electric actuation of a direct injection valve is usually carried out by means of current-regulated full-bridge output stages of the engine controller. A full-bridge output stage permits the injection valve to be supplied with an on-board power system voltage of the motor vehicle and alternatively with a boost voltage. The boost voltage (U_boost) can be, for example, approximately 60 V to 65 V. The boost voltage is usually made available by a DC/DC transformer.
FIG. 2 shows a typical current actuation profile I (thick continuous line) for a direct injection valve with a coil drive. FIG. 2 also shows the corresponding voltage U (thin continuous line) which is applied to the direct injection valve. The actuation is divided up into the following phases:
A) Pre-Charge Phase:
During this phase with the duration t_pch, the battery voltage U_bat, which corresponds to the on-board system voltage of the motor vehicle, is applied to the coil drive of the injection valve by the bridge circuit of the output stage. When a current setpoint valve I_pch is reached, the battery voltage U_bat is switched off by a two-level controller, and U_bat is switched on again after a further current threshold is undershot.
B) Boost Phase:
The pre-charge phase is followed by the boost phase. For this purpose, the boost voltage U_boost is applied by the output stage to the coil drive until a maximum current I_peak is reached. The opening of the injection valve is accelerated as a result of the rapid buildup of current. After I_peak is reached, a free-wheeling phase follows up to the expiry of t_1, and during this free-wheeling phase the battery voltage U_bat is again applied to the coil drive. The time period Ti of the electric actuation is measured starting from the beginning of the boost phase. This means that the transition into the free-wheeling phase as a result of the predefined maximum current I_peak being reached is triggered. The duration t_1 of the boost phase is permanently predefined as a function of the fuel pressure.
C) Commutation Phase:
After the expiry of t_1 there is a following commutation phase. As a result of switching off of the voltage, a self-induction voltage is produced here, which self-induction voltage is limited essentially to the boost voltage U_boost.
The commutation phase ends after the expiry of a further time period t_2.
D) Holding Phase:
The commutation phase is followed by what is referred to as the holding phase. Here, the setpoint value for the setpoint holding current I_hold is regulated by means of the battery voltage U_bat, again by means of a two-level controller.
E) Switch-Off Phase:
As a result of switching off of the voltage a self-induction voltage is produced, which self-induction voltage is, as explained above, limited to the recuperation voltage. As a result a current flow is produced through the coil, which current flow now reduces the magnetic field. After the recuperation voltage, which is shown to be a negative value here, has been exceeded, current no longer flows. This state is also referred to as “open coil”. Owing to the ohmic resistances of the magnetic material, the eddy currents which are induced during the field reduction of the coil decay. The reduction in the eddy currents leads in turn to a change in the field in the magnetic coil and therefore to a voltage induction. This induction effect causes the voltage value at the injector to rise to the value “zero” starting from the level of the recuperation voltage in accordance with the profile of an exponential function. The injector closes after the reduction of the magnetic force by means of the spring force and the hydraulic force which is caused by the fuel pressure.
The described actuation of an injection valve has the disadvantage that the times, subject to tolerances, of both the opening and closing of the injection valve or of the injector in the “open coil” phase have a negative effect on the quantity accuracy of the injected fuel.